The smell of fear is real, say scientists

Pheromones in sweat from skydivers appeared to trigger a response in brain regions associated with fear. Photograph: Solum, Stian Lysberg/AFP/Getty

The smell of fear, one of the most terrible cliches of pulp fiction, is founded in fact, scientists claim today.

People can unconsciously detect whether someone is stressed or scared by smelling a chemical pheromone released in their sweat, according to researchers who have investigated the underarm secretions of petrified skydivers.

The team found that the smell of fear triggered a heightened response in brain regions associated with fear when inhaled by volunteers in a brain scanner. The research suggests that like many animal species, humans can detect and subconsciously respond to pheromones released by other people.

The research was funded by the US Defence Advanced Research Projects Agency – the Pentagon's military research wing – raising speculation that it is a first step to isolating the fear pheromone for use in warfare, perhaps to induce terror in enemy troops. But DARPA denied that it had any military plans for fear pheromones or plans to fund further research into the field.

Dr Lilianne Mujica-Parodi at Stony Brook University in New York State and her team taped absorbent pads to the armpits of 20 novice skydivers ­– 11 men and nine women – who were doing their first tandem jump. The pads soaked up sweat before they leaped from the plane and as they fell. For comparison, the team collected sweat from the same individuals as they ran on a treadmill for a similar duration at the same time of day they had made their jump.

They transferred the two types of sweat to nebulisers and asked volunteers in a brain scanner to breathe it in. To avoid biasing the results, the team did not tell the volunteers anything about the experiment. New Scientist magazine reported that the volunteers' amygdala and hypothalamus – brain regions associated with fear – were more active in people who breathed in the "fear" sweat compared with the control. The volunteers in the brain scanner were unable consciously to distinguish between the two types of sweat and rated them both as mild and non-aversive.

In a conference presentation last year, Mujica-Parodi wrote: "We demonstrate here the first direct evidence for a human alarm pheromone … Our findings indicate that there may be a hidden biological component to human social dynamics, in which emotional stress is, quite literally, 'contagious'." She declined to comment further on the results because the study is under review with a scientific journal.

Simon Wessely, a psychiatrist at the King Centre for Military Health Research at King's College London told New Scientist that the idea that a fear pheromone could be developed as a chemical weapon is scientifically implausible. He said that a purely physiological cue is not enough to induce fear if people are not in a frightening situation. "You can generate the physical symptoms of fear but people don't necessarily get scared," he said.

The findings will be controversial because most researchers do not believe that humans can detect pheromones. In other mammals, this is done using a structure in the nose called the vomeronasal organ. Although humans have one of these it is not connected to the brain. However, human pheromones could still be detected elsewhere and some small studies have suggested that human behaviour can be modified by an alarm pheromone. In one study in 2002, for example, 60 women were asked to distinguish between sweat pads worn by women who had watched the horror film Candyman or a documentary. They rated the sweat from the scary film watchers as stronger, less pleasant and smelling more "like aggression".